This plugin takes the tag data (the text in between the tag pair) and outputs a url title (strips spaces and non-alphanumeric characters) from it.
{exp:create_url_title}
The brown cow jumps over the moon.
{/exp:create_url_title}
the_brown_cow_jumps_over_the_moon
- separator - the word separator, accepts "-", "_", or "dash", if not specified, uses your default word separator
- lowercase - set to no to prevent conversion to all lowercase